Bent probe microscopy

ABSTRACT

A general purpose force sensor for measuring nanometer scale surface topography and other characteristics includes a hollow micropipette having an inner diameter of about 7.5 nanometers at its tip. The probe includes a cantilevered structure obtained by heating it near the tip to bend it. A reflective coating is then applied to the outer surface of the micropipette.

FIELD OF THE INVENTION

The invention is a general purpose device for measuring nanometer scalesurface characteristics. The device integrally consists of a verysensitive force sensor for measuring surface topography and forces. Thestructure of the device also allows for the simultaneous monitoring of anumber of other surface characteristics. In addition, the device mayalso be used for modification and patterning with nanometer scaleresolutions.

BACKGROUND OF THE INVENTION

Scanned probe technologies today rely on a microscopically small tipinteracting with a surface as the tip is scanned in very close proximityto the surface.. The interaction between the tip and the sample cantypically be used both to track the surface topography and/or measureother characteristics. The two most common interactions which areutilized are electron tunneling (scanning tunneling microscopy--STM) andforce sensing (scanning force microscopy--SFM). Tunneling requires aconducting probe and a conducting sample and is thus restricted in itsapplication. Force sensing removes this restriction. Force sensingrequires a structure which is sensitive enough to detect the smallforces (van der walls, columbic, etc.) that are present at an interfacebetween a tip and a surface which are typically of piconewton magnitude.In addition the probe must be flexible enough so as not to deform thesurface as it scans over it. This requires a force constant on the orderof 1 Newton/meter.

One of the basic requirements of a probe for practical application ofscanned force sensing is to have a sharp, finely tapered tip which canaccurately track over and into surface corrugations. If the probe has ablunt or quickly tapered tip, scanning the tip at a constant heightabove the surface will not produce an accurate rendition of thetopography but rather a convolution of the tip structure with thesurface. This is particularly significant when force imaging is to beused for metrology applications. A great deal, of effort, usingsophisticated electron beam deposition techniques, is currently expendedin order to produce sharp enough tips for such applications Basile M.J., et al. Scanned probe tips formed by focussed ion beams, Rev. sci.instrm. 62,2167 (1991); Kado H., Yokoyama K. and Tohda T. A Novel ZnOWhisker Tip for Atomic Force Microscopy, Ultramicroscopy (1992)!.

The deflection of the probe induced by the interaction with the surfaceforce is generally detected by optical means. For the detection ofnormal forces of the surface on the tip, the probe consists of acantilever with a tip hanging off one end. The forces on this tip aretypically measured by focusing a laser beam onto a small spot on theback side of the probe. When the probe bends the small angular deviationof the beam is detected with a position sensitive detector D. Rugar andP. K. Hansma, Phys. Today, 43, 23 (1990); K. Wickramasinghe, Sci. Am.26, 90 (1989)!. Alternatively, the motion of the probe and beam can bemonitored interferometrically D. Rugar and P. K. Hansma, Phys. Today 43,23 (1990); K. Wickramasinghe, Sci. Am. 26, 90 (1989)!. Both techniquesrequire for normal force sensing a small, flat reflecting surface onwhich to direct the beam. Alternately, lateral force sensing does notdepend on a cantilevered structure.

STATE OF PRIOR ART

The first normal force cantilevers were fabricated by etching thin wiresand mechanically bending them near the tip to produce a cantileveredstructure D. Rugar and P. K. Hansma, Phys. Today 43, 23 (1990); K.Wickramasinghe, Sci. Am. 26, 90 (1989)!. Such probes had a number ofproblems including control over etching and the difficulty inmechanically bending the tip in a reproducible fashion. In addition suchprobes are not well suited to optical deflection sensing since theycontain no flat region which may be used to reflect a laser beam.

Force cantilevers in common use today are typically microfabricated withconventional microlithography techniques D. Rugar and P. K. Hansma,Phys. Today 43, 23 (1990); K. Wickramasinghe, Sci. Am. 26, 90 (1989)!.Such probes consist of a thin silicon membrane or cantilever onto whicha small sharp cone is produced D. Rugar and P. K. Hansma, Phys. Today43, 23 (1990); K. Wickramasinghe, Sci. Am. 26, 90 (1989)!. At the tip ofthe cone an additional filament is often grown to produce a sharper andfiner tapered tip. The mechanical characteristics of such probes aredetermined by the materials used, tip mass and geometry. Typical forceconstants for such tips are in the 0.1 to 10 Newton/Meter range. Theseprobes, however, are not very suitable for other forms of scanned probemicroscopy.

SUMMARY OF THE INVENTION

The invention is a method and a device for producing a general purposeprobe for all forms of scanned probe imaging and patterning. Thestructures produced with this method are immediately compatible with allthe force deflection sensing techniques in use today.

The device is based on a glass or quartz micropipette or fiber which canbe pulled down to a variety of dimensions at the tip with 10 nanometersthe smallest outer diameter achieved thus far. The micropipettes remainhollow and can have an inner diameter at the tip of 7.5 nanometers.These probes may be pulled with a very gradual taper giving a cone angleat the tip of only a few degrees, or can be tapered with larger coneangles if the application so requires.

For lateral force sensing nothing further needs to be done. For theaddition of normal force sensing the probe is then given a cantileveredstructure by locally heating it near the tip and applying a small forceto bend the tip when the glass or quartz becomes soft. This is shownschematically in FIG. 1. Localized heating is achieved by focusing a COlaser to a small spot near the tip. A stream of air is directed at thetip region while the heating is taking place. This serves two purposes.First, the air cools the tip so that there will not be excessive heatconduction in the glass which could melt the tip of the probe andsecond, the air flow provides sufficient force to bend the very tip assoon as the glass or quartz becomes sufficiently soft. The bend radiusis determined principally by the size of the laser heating spot.Focusing a CO₂ laser down to a diffraction limited 10μ spot can readilyproduce bends a few 10's of microns from the tip.

Once the cantilevered structure is obtained the crucial polishing steptakes place to provide the incorporation of optical deflection sensingtechniques. This is done by inserting the cantilevered structure into amicromanipulator and bringing the end of the bent region into contactwith a rotating polishing surface as shown in FIG. 2. The polishedregion may be as small as several microns in diameter. This produces anoptically flat surface from which a laser beam may be reflected tomonitor the deflection of the cantilever. This polished region is justabove the bent section of the tip for maximum deflection sensitivity.

After the tip is bent a reflecting metallic coating may be deposited onthe polished section to enhance the reflectivity. A metal coating may befurther applied to the entire outer region of a micropipette probe andthe walls of a fiber probe to produce a structure which may also be usedas a near-field subwavelength point of light.

Such bent polished structures can simultaneously be used to pass lightthrough the device to the tip which, as noted above, can be transformedinto a near-field aperture by an appropriate metal coating. With bentfibers light is guided around the bend. With pipettes a high indexliquid can be made to fill the pipette void and this also permits thetransmission of light around the bend and through the near-field,sub-wavelength aperture at the tip. The resulting sub-wavelength pointof light can be used for imaging and patterning while force is used,either normal or lateral, to monitor the topography and forcecharacteristics of the surface. Furthermore, in such a structure themetallic coating at the tip can be used to measure simultaneouslytunneling currents to determine the tunneling characteristics. Withmicropipettes these same structures can be readily filled with anoptically or electrically excited light-emitting substance to produce asub-wavelength source of light with many of the above sensingcapabilities. The light emitting substance at the tip of the pipette canalso be used to monitor specific ions or sense surface charge.Alternately, micropipette structures can be produced with a metal wiredown the hollow interior extending to the tip. The coating of metal onthe outside of this structure if it is generated using a different metalfrom that in the interior, permits the production of a thermal sensorwith the force and tunneling characteristics noted above. Such a metalsandwiched glass structure with a transparent glass tip could also beused to propagate light without evanescent losses in the subwavelengthregion. This structure would be the optical analog to an electricalcoaxial cable. As another alternative sol-gel conducting glass can alsobe deposited in the tip and this glass can be embedded with opticallyexcited materials to produce a structure which could monitor optical,electrical and the conductive nature of surfaces. The essence of allthese structures are the multichannel sensing capabilities which havebeen patently absent in scanned probe microscopes because no elementssuch as the ones noted here have been devised.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing, and additional objects, features and advantages of thepresent invention will become apparent to those of skill in the art fromthe following detailed description of a preferred embodiment thereof,taken with the accompanying drawings, in which:

FIG. 1 is a diagrammatic illustration of apparatus for bending amicropipette;

FIG. 2 illustrates apparatus for producing a beveled, polished surfaceon a bent micropipette; and

FIG. 3 is a cross-sectional view of a micropipette in accordance withthe invention.

DETAILED DESCRIPTION

A cross-sectional view of a bent probe produced according to the presentinvention is shown in FIG. 3. It consists of a glass or quartzmicropipette or a glass or quartz optical fiber 1.1. The micropipette istapered to produce a hole at the tip that can be as small as 7.5 nm,whilethe glass or quartz optical fiber is tapered to produce a tip outerdiameter that can be as small as 10 nanometers. The micropipette orfiber is secured in a micromanipulator 2.0 as illustrated in FIG. 1, andis bent, for normal force sensing, near the tip by heating it locally atregion 2.1 with a laser 2.2 such as a carbon dioxide laser. A stream ofair may be directed toward the tip region to cool it and to bend the tipportion 2.3 to produce a cantilevered bent probe structure. Thereafter,the bent micropipette or fiber 1.1 is polished just above the bend, asillustrated in FIG. 2, by a polishing surface 2.4 to produce anoptically flat bevelled surface 1.2.

A material 1.3 such as aluminum or gold is optionally deposited alongthe outer surface of the probe to provide for an opaque coating ifrequired.

A reflective coating 1.4 such as aluminum is deposited on the beveled,polished surface of part 1.1 to provide a reflecting surface.

A material 1.5 is optionally inserted into the very tip of themicropipetteprobe which acts as a specific chemical, optical or thermalsensor for various local phenomenon.

The bent probe is incorporated into a micropositioning instrument, andcan be inserted with micropositioner under the lens of a regularmicroscope which can be used together with an interferometricmeasurement through this lens to sense the deflection the micropipettecantilever with the lens also being used for collection of light fromthe sample and illumination of the sample.

In operation, the probe is brought into the near field of the surface byeither monitoring lateral force in a non-bent structure or by monitoringthe deflection of the cantilever in a bent structure by normal forceimpinging on the tip of the bent pipette or fiber. Then the structure isscanned along the surface either in contact or in near-contact bymonitoring surface forces while the other attributes of the tip are usedto monitor simultaneously the chemical, optical, electrical or thermalcharacteristics of the surface.

The probes produced with the technique described here have a number ofadvantages over presently available scanned force probes. First, theinitial pulling technique allows for simply and accurately controllingandadjusting the force characteristics and constants of the probeitself. Second, the pulling process naturally produces very sharp andfinely tapered tips which are required for accurate force imaging.Conventionallyproduced tips require a complex and poorly understoodgrowth of a fine filament, with the aid of electron beam depositiontechniques, at the tip of the microfabricated cone as an additional stepafter the sensor is completed. Finally the micropipette and fiber probesallow numerous other surface characteristics to be monitoredsimultaneously with force sensing.This includes near-field opticalinteractions and, in the case of the micropipette probe a variety ofspecific sensors, may be placed within thetip.

We claim:
 1. A probe, comprising:a tapered micropipette having a hollowtip drawn to an inner tip diameter of 10 nanometers or less, saidmicropipette having a bend near said tip and mounted to produce acantilevered bent probe structure suitable for normal force sensing; andan optically flat polished region near said, bend for monitoringdeflection of the cantilevered structure.
 2. The probe of claim 1,wherein said micropipette includes an outer wall surface coating toproduce a probe suitable for near-field scanning optical microscopy andlithography.
 3. The probe of claim 2, further including a materialinserted in said hollow tip to act as a specific chemical,spectroscopic, surface charge, or other sensor of a local environment atsaid tip.
 4. The probe of claim 3, wherein said material in said hollowtip and said outer surface coating are different metals selected toproduce a highly localized thermocouple at said tip.
 5. The probe ofclaim 1, further including:a micromanipulator for mounting said bentprobe under the lens of a microscope for interferometric measurementthrough the lens to sense the deflection of the bent probe by motion ofsaid flat polished region while using the lens for illumination of asample and collection of light from the sample.
 6. A probe, comprising:atapered optical fiber having a tip drawn to an outer tip diameter of 10nanometers or less, said fiber having a bend near said tip and mountedto produce a cantilevered bent probe structure suitable for forcesensing; and an optically flat polished region near said bend formonitoring deflection of the cantilevered structure.
 7. The probe ofclaim 6, including:a micromanipulator for mounting said bent probe underthe lens of a microscope for interferometric measurement through thelens to sense the deflection of the bent probe by motion of said flatpolished region while using the lens for illumination of a sample andcollection of light from the sample.
 8. A method for producing ultrafinecantilevered glass or quartz micropipette or optical fiber probes withforce sensing characteristics, comprising:drawing a micropipette orfiber to produce a tip having a tapered end portion having a minimumdimension of 10 nm or less; heating the tip at a location a few tens ofmicrons from the tapered end portion by heating; and bending the tip atthe heated location.
 9. The method of claim 8, further includingpolishing said tip at a surface location near the location of bending toproduce an optically flat polished region for monitoring deflection ofthe tip.
 10. A method for producing an optically flat reflecting surfaceon a cantilevered micropipette or fiber bent probe, comprising:holding abent probe in a micromanipulator; and polishing the outer surface of thebent probe using a polishing wheel to provide a reflecting surface toallow monitoring of probe deflection using optical deflection sensingtechnology.